Infrared reflection-absorption spectroscopy

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The infrared reflection absorption spectroscopy ( IRRAS ) is a sampling technique of infrared spectroscopy for non-destructive examination of thin layers . It is a hybrid of transmission and reflection infrared spectroscopy and is therefore also known as transflexion (especially for thicker samples) .

In addition to IRRAS, other terms are used synonymously for this technology. You can find the abbreviation IRAS or modified forms in which the order of the word parts is varied (IRRS, RAIR, RAIRS, RAS etc.) or it is pointed out that a Fourier transform infrared spectrometer (FTIR spectrometer) is used (FT- IRRAS, FT-IRAS, FTIR / RA etc.) or it is an external (external) reflection (IR-ERS, ERIR etc.; in contrast to the internal reflection, as used in ATR spectroscopy ) or grazing incidence acts (GIR, G stands for grazing = 'grazing'). Even if encountered in English-language publications abbreviation RAIRS ( English reflection-absorption infrared spectroscopy ) from the from the International Union of Pure and Applied Chemistry is recommended (IUPAC), the abbreviation frequently used in German publications to in this article IRRAS be used.

background

The technique most frequently used in the infrared spectroscopic examination of samples is the measurement in transmission . A sample is irradiated with infrared light. The spectrum obtained can be compared with a reference spectrum, and a spectrum is obtained either in transmission or in absorption (one speaks of transmission or absorption spectrum) which contains information on the absorption centers of the sample in the form of simple intensity curves . The degree of transmission is directly proportional to the amount of substance irradiated (results from the concentration and thickness of the sample). Since there is a general weakening of the radiation intensity during radiation, the transmission technology is limited to transparent and weakly to moderately absorbing materials, as otherwise not enough infrared light will reach the detector.

For the measurement of "more difficult" samples, for example strongly absorbing samples, other sampling techniques must be used, such as measuring the sample in (directed, external) reflection. Pure reflection spectra differ significantly in their shape from the transmission spectra. They show complex intensity gradients in the area of ​​the absorption centers, which are comparable to poles . The conversion between reflection and transmission spectra is possible using the Kramers-Kronig transformation . These converted spectra are only rarely suitable for a direct comparison of the spectra, for example for the determination of a substance through a database search.

functionality

IRRAS is a combination of transmission and reflection measurement in which a thin layer or an adsorbate is measured on a reflective, often metallic substrate , in which the infrared radiation is reflected on the substrate and the sample layer is irradiated twice. The double sample run that occurs and the associated larger absorption signal is a side effect of this technique. Depending on the sample (degree of absorption and layer thickness) it is beneficial to adjust the angle of incidence. For layers 0.5–20 µm thick, the angle of incidence is usually between 10 ° and 60 °. Layers in the range of a few nanometers, on the other hand, can best be measured at grazing incidence (angle of incidence greater than 80 °).

If the layer thickness of the substances to be examined is in the range of a few nanometers, the use of very sensitive semiconductor detectors (usually so-called MCT detectors , from English mercury cadmium telluride , dt. Mercury-cadmium-telluride ) is usually necessary for IRRA spectroscopy . necessary.

Functional principle of PM-IRRAS using the example of adsorbed carbon monoxide on a metal surface. The CO group oscillating perpendicular to the surface can only interact with p-polarized radiation.

The IRRAS technology can also be used with polarized radiation. The advantage here is the fact that infrared radiation that is linearly polarized perpendicular to the plane of incidence has a negligibly low field strength in the area of ​​the interface of the metallic substrate and virtually does not interact with the dipoles of a substance adsorbed on the surface. If the sample is now alternately irradiated with perpendicular and parallel linearly polarized radiation (for example by using a photoelastic modulator ), the spectrum shows a change between the reference spectrum without adsorbate (perpendicularly polarized) and the spectrum with adsorbate (parallel polarized). The real-time recording of the reference and sample spectrum makes it easy to minimize interference from carbon dioxide (CO 2 ) or water vapor (H 2 O) from the beam path. However, the spectrum of the adsorbate only shows interactions with dipoles parallel to the plane of incidence, since only these interact with the parallel polarized radiation. So that the difference between these two states becomes clear, the sample is measured under grazing incidence (approx. 80 °). This technique, known as polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS), enables the adsorption of substances to be investigated in a relatively simple manner.

Advantages and disadvantages

Compared to reflection spectra, IRRA spectra are more similar to transmission spectra, even if both are only comparable to a limited extent (see interference effects). This is an advantage in practice, because measurement in transmission has long been the dominant sample technique, for which more extensive spectrum catalogs and databases exist than for other sample techniques such as reflection or IRRAS. Due to the relatively good comparability with "transmission spectra", these databases can also be used for the IRRAS technology.

In the case of layer thicknesses below the wavelength used, standing waves and thin-layer interference effects can develop, as with transmission measurements, which influence the analysis of the spectrum, make it very difficult or impossible. Due to the strong reflection of the metallic layer, these effects are much greater than with transmission measurements. With thinner layers (below a quarter of the wavelength) the Lambert-Beer law no longer applies . This means that the transmission / absorption is no longer only dependent on the absorption coefficient and the layer thickness, but also on field variations in the vicinity of the reflective, metallic surface. As already described, this can be used to achieve a sub- monolayer sensitivity, which in the best case enables the characterization of layers smaller than 10 −4 monolayers. Furthermore, the use of polarization-dependent measurements increases the complexity of the investigation, especially since they are angle-dependent. This can be disadvantageous for quick standard examinations, but it also enables specific processes to be analyzed more precisely. For thicker layers, the deviations from the Lambert-Beer law become increasingly stronger. In addition to changes in the relative peak intensities, there are shifts in the peaks before additional peaks can finally occur due to the interference effects. In principle, these effects can be corrected.

application

IRRAS was first used at the end of the 1960s by Robert G. Greenler to investigate organic layers on metallic mirrors. He also dealt with the theoretical background of this measurement technique. Since then, the IRRAS technology has been improved in many areas. Above all, however, the improvement of the signal-to-noise ratio of the infrared spectrometer has contributed to the fact that IRRAS is now used in a variety of ways for the investigation of thin layers on and surface reactions on smooth, mostly metallic surfaces; both metal-gas and metal-liquid interfaces.

Typical applications are the characterization of substances adsorbed on metal surfaces, their changes and the reaction kinetics, e.g. for carbon monoxide (CO). The analysis of catalytic reactions or the electrode-electrolyte interface is also often carried out using IRRAS, as is the investigation of thin, dielectric layers in semiconductor technology.

literature

  • Peter R. Griffiths, James A. De Haseth, James D. Winefordner: Fourier Transform Infrared Spectrometry . 2nd Edition. Wiley John + Sons, 2007, ISBN 0-471-19404-2 , pp. 277-300 .
  • Peter Hollis: Infrared Reflection-Absorption Spectroscopy . In: Robert A. Meyers (Ed.): Encyclopedia of Analytical Chemistry: Applications, Theory, and Instrumentation . John Wiley & Sons, 2000, ISBN 0-471-97670-9 .

Individual evidence

  1. ↑ Rudolf W. Kessler: Process Analytics: Strategies and Case Studies from Industrial Practice . Wiley-VCH, 2006, ISBN 978-3-527-31196-5 , pp. 231 .
  2. ^ Peter R. Griffiths, James A. De Haseth, James D. Winefordner: Fourier Transform Infrared Spectrometry . 2nd Edition. Wiley John + Sons, 2007, ISBN 0-471-19404-2 , pp. 297-300 .
  3. ^ Ricardo Aroca: Surface-Enhanced Vibrational Spectroscopy . John Wiley & Sons, 2006, ISBN 0-471-60731-2 , pp. 60 .
  4. ↑ a b c Peter Hollis: Infrared Reflection Absorption Spectroscopy . In: Robert A. Meyers (Ed.): Encyclopedia of Analytical Chemistry: Applications, Theory, and Instrumentation . John Wiley & Sons, 2000, ISBN 0-471-97670-9 .
  5. ↑ Examples of spectra in: Peter R. Griffiths, James A. De Haseth, James D. Winefordner: Fourier Transform Infrared Spectrometry . 2nd Edition. Wiley John + Sons, 2007, ISBN 0-471-19404-2 , pp. 284-285 .
  6. ↑ Robert G. Greenler: Infrared Study of Adsorbed Molecules on Metal Surfaces by Reflection Techniques . In: The Journal of Chemical Physics . tape 44 , 1966, doi : 10.1063 / 1.1726462 .
  7. ^ A. Neckel: In situ investigations of the solid / solution interface . In: Fresenius' Journal for Analytical Chemistry . tape 319 , 1984, doi : 10.1007 / BF01226750 .
  8. ↑ a b The electric field standing wave effect in infrared transflection spectroscopy . In: Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy . tape 191 , February 15, 2018, ISSN  1386-1425 , p. 283–289 , doi : 10.1016 / j.saa.2017.10.033 ( sciencedirect.com [accessed November 6, 2018]).
  9. ^ Thomas G. Mayerhöfer, Harald Mutschke, Jürgen Popp: The Electric Field Standing Wave Effect in Infrared Transmission Spectroscopy . In: ChemPhysChem . tape 18 , no. 20 , August 24, 2017, ISSN  1439-4235 , p. 2916–2923 , doi : 10.1002 / cphc.201700688 ( wiley.com [accessed November 6, 2018]).
  10. ^ Günter Gauglitz , Tuan Vo-Dinh (ed.): Handbook of Spectroscopy . Wiley-VCH Verlag, 2003, ISBN 3-527-29782-0 , pp. 75, 558-561 .
  11. ^ Thomas G. Mayerhöfer, Susanne Pahlow, Uwe Hübner, Jürgen Popp: Removing interference-based effects from the infrared transflectance spectra of thin films on metallic substrates: a fast and wave optics conform solution . In: The Analyst . tape 143 , no. 13 , 2018, ISSN  0003-2654 , p. 3164-3175 , doi : 10.1039 / C8AN00526E ( rsc.org [accessed November 6, 2018]).
  12. ↑ according to Richard C. Alkire, Dieter M. Kolb , Jacek Lipkowski, Phil Ross: Diffraction and Spectroscopic Methods in Electrochemistry . Wiley-VCH, 2006, ISBN 3-527-31317-6 , pp. 315 .
    • Robert G. Greenler: Infrared Study of Adsorbed Molecules on Metal Surfaces by Reflection Techniques . In: The Journal of Chemical Physics . tape 44 , 1966, pp. 1963 , doi : 10.1063 / 1.1726462 .
    • Robert G. Greenler: Reflection Method for Obtaining the Infrared Spectrum of a Thin Layer on a Metal Surface . In: The Journal of Chemical Physics . tape 50 , 1969, p. 310 , doi : 10.1063 / 1.1671315 .
    • Robert G. Greenler: Design of a reflection – absorption experiment for studying the ir spectrum of molecules adsorbed on a metal surface . In: Journal of Vacuum Science and Technology . tape 12 , 1975, p. 1410 , doi : 10.1116 / 1.568552 .